Multichannel wide dynamic-range compression (WDRC) processing has become the norm in modern digital hearing aids. WDRC can be considered in the light of two contradictory signal-processing assumptions. One assumption is that compression amplification will improve speech intelligibility because it places more of the speech above the impaired threshold. The opposing assumption is that compression amplification will reduce speech intelligibility because it distorts the signal envelope, reducing the spectral and temporal contrasts in the speech. The first assumption is used to justify fast time constants (syllabic compression) and more compression channels, while the second assumption is used to justify slow time constants (automatic gain control, or AGC) and fewer channels. Fast compression and a large number of narrow frequency channels maximizes audibility but increases distortion, while slow compression using a reduced number of channels minimizes distortion but provides reduced audibility.
An additional assumption in most WDRC systems is that the entire audible intensity range must be compressed to fit within the residual dynamic range of the hearing-impaired listener. This assumption, for example, is the basis of compression systems that use loudness scaling in an attempt to match the loudness of sound perceived by the impaired ear to that perceived by a normal ear.
A hearing aid with a compressor having a low and gain independent delay and low power consumption is disclosed in EP 1 448 022 A1.
A hearing aid with a compressor in which attack and release time constants are adjusted in response to input signal variations is disclosed in WO 06/102892 A1.
A summary of previous compression studies has shown that there are many conditions where linear amplification yields higher intelligibility and higher speech quality than compression. Simulation results indicate that linear amplification give higher intelligibility and higher quality than compression as long as the speech is sufficiently above the impaired threshold. Compression gives substantially better predicted intelligibility and quality only for the condition of low signal levels combined with a moderate/severe hearing loss.